Energy storage elements such as lithium ion secondary batteries are used as power sources for mobile devices such as notebook computers and mobile phones. In recent years, they have come to be used as power sources for industrial applications such as rail vehicles and industrial machines. In such power sources for industrial applications, a plurality of energy storage units, each including a plurality of energy storage elements connected in series, are connected in parallel so as to achieve a high capacity.
In this case, capacity imbalance occurs among the plurality of energy storage units, and it is therefore important to eliminate the capacity imbalance. For this reason, conventionally, a technique has been proposed for eliminating the capacity imbalance among a plurality of energy storage units connected in parallel by performing control so as to uniform the remaining capacity of the plurality of energy storage units (see, for example, Japanese Unexamined Patent Application Publication No. 2011-72153).